A charged particle beam exposure apparatus, such as an electron beam exposure apparatus, is typically used to direct write a pattern on the surface of a mask blank or substrate during semiconductor processing. Blanking aperture arrays can be used with a charged particle beam exposure apparatus to separate the charged particle beam into multiple beams that are used to generate an array of exposure pixels on the substrate surface as discussed in the article entitled "Multielectron beam blanking aperture array system SYNAPSE-2000," by Hiroshi Yasuda et al., J. Vac. Sci. Technol. B. 14(6), pp. 3813-3820, Nov/Dec 1996, which is incorporated herein by reference. As is well known in the art, individual apertures of a blanking aperture array can be "opened" or "closed" with an electrical potential applied to deflection plates near each aperture. By controlling which apertures are open or closed to the charged beam, the blanking aperture array may be used to generate the desired pattern of exposure pixels on a substrate surface.
FIG. 1 is a cross-sectional view of a simplified charged particle beam column 10 using a blanking aperture array to generate individual beams. Charged particle beam column 10 is an electron beam exposure apparatus, however, it should be understood that other charged particles, e.g., ions, may alternatively be used. Column 10 is conventionally within a vacuum enclosure.
Column 10 includes an electron source 12 that conventionally generates a electron beam 14 in alignment with an optical axis A. An electrostatic or magnetic electron lens 16 flood-illuminates a blanking aperture array ("BAA") 20 with approximately parallel electron beams 18. Although lens 16 is shown as a single lens, a lens system is often used to generate the desired flood illumination. Typically, the electron source, which is approximately 10 .mu.m in diameter, requires a large magnification if a large area of BAA 20 is illuminated, which necessitates a large column length.
BAA 20 includes a number of small apertures 20a, which correspond to the desired exposure pixels projected on the writing plane of substrate 28. Typically apertures 20a of BAA 20 are approximately 10 to 50 .mu.m (micrometers) in diameter or per side if square and one to several aperture sizes apart.
Electron beams 24 pass through each aperture 20a of BAA 20. Electrostatic deflectors 22 are used to "close" individual apertures of BAA 20 by deflecting the electron beam passing through a particular aperture with an electric potential, thereby preventing the electron beam from reaching the writing plane (surface, e.g. of resist) of substrate 28. Electrostatic deflectors 22 are controlled to permit electron beams 24 to pass in accordance with the desired exposure pattern.
Electron beams 24 are then focused with a lens 26, which may be a lens system, onto the writing plane of substrate 28. Because apertures 20a of BAA 20 are approximately 10 .mu.m in diameter and therefore electron beams 24 are approximately 10 .mu.m in diameter, electromagnetic lens 26 must demagnify electron beams by at least 100 to achieve a resolution of 100 nm (nanometers) on the writing plane on substrate 28.
The large magnification and demagnification used in charged particle beam column 10 typically requires a complex lens system and a long column length. The complex lens system generally used in charged particle beam column 10 is not shown in FIG. 1 for the sake of clarity. A typical length of a conventional charged particle beam column 10 is approximately 0.5 m. In a long column, electron-electron interactions increase the beam blur in each of the electron beams, which consequently results in lower throughput.
Additional spurious effects are caused by flood illumination of BAA 20 with electron beams 18, such as loss of efficiency, heating, and charging. FIG. 2 is a perspective view of BAA 20 with electron beams 18 flood illuminating apertures 20a as well as an area on the surface of BAA 20. As shown in FIG. 2, a large amount of flood illumination 18 is lost illuminating the surface of BAA 20 and only a small percentage of flood illumination 18 is actually passed through apertures 20a, resulting in a transmission efficiency of only a few percent. Consequently, a large amount of energy from flood illumination 18 is absorbed by the surface of BAA 20 itself, rather than being transmitted through apertures 20a. The absorbed energy from flood illumination 18 produces heating of BAA 20 resulting in expanding aperture diameters and thermal drift, i.e., uncontrolled shift of apertures with respect to optical axis A and relative to one another. Further, flood illumination 18 that is incident on the surface of BAA 20 often results in undesirable charging of BAA 20.
In addition, flood illumination 18 may generate scattering of electrons off the edges and side walls of apertures 20a resulting in undesirable resist exposure. FIG. 3 is a cross-sectional view of a single aperture 20a of BAA 20 with a single electron ray path 30, illustrated as an arrow. Typically, the electron beams 18 used to flood illuminate of BAA 20 will contain electron ray paths that are not perfectly perpendicular with BAA 20. Thus, as shown in FIG. 3, an electron beam 30 may be scattered at an angle by the edge or a sidewall of aperture 20a. When a large number of electrons are scattered, the beam may become blurred which reduces resolution and decrease pattern fidelity.